The invention relates generally to the field to optical measurements and measuring systems, and more particularly to a system and method for optical heterodyne detection of an optical signal.
Optical heterodyne detection systems are utilized to analyze an optical signal. FIG. 1 is a depiction of a prior art optical heterodyne detection system that includes an optical coupler 110 that combines an input signal 102 from an input fiber 104 with a local oscillator signal 106 from a local oscillator fiber 108. The combined optical signal travels on an output fiber 118 and is detected by a photodetector 112. The photodetector converts optical radiation from the combined optical signal into an electrical signal. The electrical signal is processed by a signal processor 116 to determine characteristics of the input signal, such as wavelength and amplitude. In order to optimize optical heterodyne detection, it is important that the polarization of the input signal and the local oscillator signal are matched. In order to match the polarization of the local oscillator signal to the polarization of the input signal, the local oscillator signal includes a polarization controller 120 as indicated by the two loops in the local oscillator optical fiber. A disadvantage of the optical heterodyne detection system of FIG. 1 is that detection of the input signal is highly dependent on the polarization of the input signal.
A polarization diversity receiver can be incorporated into an optical heterodyne detection system to provide polarization independent signal detection. FIG. 2 is a depiction of an optical heterodyne detection system that incorporates a polarization diversity receiver. Throughout the specification, similar elements are identified by similar element numbers. The optical heterodyne detection system includes a polarization controller 220 on the local oscillator fiber 208, an optical coupler 210, a polarizing beam splitter 224, two photodetectors 212 and 214, and a processor 216. The polarizing beam splitter splits the combined optical signal into two polarized beams that are separately detected by the respective photodetectors. The polarized beams that are detected by the two photodetectors include an intensity noise component and a heterodyne component, as is known in the field of optical heterodyne detection. The heterodyne components of the polarized beams are utilized to determine the desired characteristics of the input optical signal, such as wavelength and amplitude.
Processing of the electrical signals generated by the two photodetectors 212 and 214 involves squaring the electrical signals generated from the two polarized beams, low pass filtering the squared terms, and then adding the filtered terms together. Although the polarization diversity receiver provides polarization independent signal detection, the polarization diversity receiver does not provide a way to separate the intensity noise components from the heterodyne components of the combined optical signal. In order to improve the performance of heterodyne detection systems with regard to parameters such as sensitivity and dynamic range, it is necessary to be able to clearly distinguish the heterodyne components from the intensity noise components of the combined optical signal that includes the input signal and the local oscillator signal.
In view of the prior art limitations, what is needed is an optical heterodyne detection system that provides polarization independence and intensity noise suppression.
A system and method for optical heterodyne detection of an optical signal includes an optical coupler and a polarizing beam splitter that split a combined input signal and local oscillator signal into four polarized beams. The four polarized beams are detected by four photodetectors that generate four different electrical signals in response to the four polarized beams. The four electrical signals are then processed to provide an output response that is independent of the polarization state of the original input signal and in which the intensity noise has been suppressed. Processing of the electrical signals to suppress the intensity noise involves canceling the intensity noise component of similarly polarized beams by subtracting signals related to the similarly polarized beams. Processing of the electrical signals to provide polarization diversity involves squaring the value generated from the two subtraction operations, low pass filtering the squared values, and then adding the filtered values together.
An embodiment of an optical heterodyne detection system includes an input signal and a local oscillator signal that are combined in an optical coupler to create a combined optical signal. The optical coupler includes two outputs for outputting a first beam and a second beam of the combined optical signal. A polarizing beam splitter is optically connected to the optical coupler in order to receive the first beam and the second beam. The polarizing beam splitter includes four outputs for outputting four beams including two polarized portions of the first beam and two polarized portions of the second beam. Four photodetectors are optically connected to the polarizing beam splitter to receive a different one of the four beams. The four photodetectors generate four electrical signals in response to respective ones of the four beams.
In an embodiment, the first photodetector corresponds to an ordinary portion of the first beam and the second photodetector corresponds to an extraordinary portion of the first beam. The third photodetector corresponds to an ordinary portion of the second beam and the fourth photodetector corresponds to an extraordinary portion of the second beam. In order to suppress intensity noise related to the four beams, the system further includes a processor for subtracting an electrical signal generated from the first photodetector from an electrical signal generated from the third photodetector, thereby creating a first subtracted signal, and for subtracting an electrical signal generated from the second photodetector from an electrical signal generated from the fourth photodetector, thereby creating a second subtracted signal.
In order to create an output signal that is independent of the polarization of the original input signal, the processor squares the first subtracted signal, thereby creating a first squared signal, squares the second subtracted signal, thereby creating a second squared signal, filters the first and second squared values with low pass filtering, thereby creating first and second filtered signals, and then adds the first filtered signal to the second filtered signal.
Before utilizing the system to measure an input signal it may be necessary to calibrate the system. A switch may be included with the system to block the input signal so that the optical coupler, the polarizing beam splitter, and the responsivity of the photodetectors can be calibrated.
A method for monitoring an optical signal utilizing optical heterodyne detection involves combining an input signal with a local oscillator signal and outputting a first beam and a second beam of the combined optical signal. The first beam is split into a first split beam having a first polarization state and into a second split beam having a second polarization state. The second beam is split into a third split beam having the first polarization state and into a fourth split beam having the second polarization state. The first split beam is detected and a first electrical signal is generated in response to the first split beam. The second split beam is detected and a second electrical signal is generated in response to the second split beam. The third split beam is detected and a third electrical signal is generated in response to the third split beam. The fourth split beam is detected and a fourth electrical signal is generated in response to the fourth split beam. The first, second, third, and fourth electrical signals are processed in order to suppress the intensity noise components of the four beams and to achieve polarization independence.
In an embodiment, the processing involves subtracting the first electrical signal from the third electrical signal in order to suppress intensity noise, thereby creating a first subtracted signal, subtracting the second electrical signal from the fourth electrical signal in order to suppress intensity noise, thereby creating a second subtracted signal, squaring the first subtracted signal, thereby creating a first squared signal, squaring the second subtracted signal, thereby creating a second squared signal, low pass filtering the first and second squared signals, thereby creating first and second filtered signals, and adding the first filtered signal to the second filtered signal in order to achieve polarization independence .
The optical heterodyne detection system and method provide an optical measurement system that is accurate over a wide range of wavelengths. The optical heterodyne detection system and method can be utilized as an optical spectrum analyzer to characterize an unknown input signal. The optical heterodyne detection system and method may also be utilized as an optical network analyzer in which a known signal is input into an optical network and the output signal is measured by the detection system.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.